1. Field of Invention
The invention relates to a system and a control device thereof, and in particular, to a fan system and a control device thereof.
2. Related Art
Because the performance and the processing speed of electronic devices are increased and the integration of the used electronic components is increased, the requirement in heat dissipation is greatly increasing. In one solution, a fan is installed in the electronic device to dissipate the heat. A control device of the fan adjusts the rotating speed of the fan according to the environment temperature. The environment temperature is usually sensed by a temperature sensing member, such as a thermister. The thermister is a resistor, which is sensitive to the temperature variation and changes its resistance value with the temperature variation. The thermisters may be classified into a positive temperature coefficient (PTC) thermister and a negative temperature coefficient (NTC) thermister under the correlation between the temperature and the resistance value. The resistance value of the PTC thermister increases when the temperature rises, while the resistance value of the NTC thermister decreases when the temperature rises. The thermisters have been widely used in the electronic devices to serve as temperature sensing members to facilitate the control and compensation of the circuit.
Referring to FIG. 1, a conventional fan system 1 includes a sensing module 11, a driving module 12 and a motor 13, and the sensing module 11 has a resistor R and a thermister RTH. The resistor R has a first end for receiving a working voltage V1 of the driving module 12, and a second end electrically connected with one end of the thermister RTH. The thermister RTH changes its resistance value according to the variation of an external temperature. The thermister RTH and the resistor R divide the working voltage V1 to generate a sensing signal S1, which is transferred to the driving module 12. The driving module 12 generates a control signal S2 according to the sensing signal S1, which is a voltage value, to control the rotating speed of the motor 13. In this example, the driving module 12 is a chip and has a plurality of pins including a pin VREG for providing the working voltage V1 and a pin VTH for receiving the sensing signal S1.
However, when the fan system 1 is being operated, assembled or installed, the thermister RTH may be detached from the fan system 1 or broken due to the influence of vibrations or external forces. Thus, the fan system 1 cannot change the rotating speed with the temperature variation, and the motor 13 keeps the original rotating speed. If the fan system 1 continues operating, the temperature gradually rises but the motor 13 still keeps the same rotating speed. Thus, the fan system 1 is overheated and thus damaged, the reliability and the use efficiency are decreased, and the cost is increased because the fan system has to be changed frequently.
At present, in order to improve the above-mentioned method, when the thermister RTH is detached or broken, the rotating speed of the motor 13 is configured to be the full speed. To achieve this object, the thermister RTH having the resistance value, such as 5KΩ, has to be selected, and the resistance value of the resistor R has to be smaller than 5KΩ. Thus, when the thermister RTH is detached or broken, the current flowing into the driving module 12 (VTH) is high enough because the resistance value of the resistor R is very small. Thus, the driving module 12 can generate the control signal S2 to control the motor 13 to rotate at the full speed. The resistance value of the thermister RTH required in this method is smaller, so the selection is limited.
As shown in FIG. 2, another conventional fan system 1A is to supply a power V2 with 12 volts to the sensing module 11. The motor 13 is controlled to rotate at the full speed when the thermister RTH is detached or broken regardless of the amounts of the resistance values of the thermister RTH and the resistor R. However, when the thermister RTH is not detached or broken, its resistance value gets larger as the sensed temperature gets lower. Thus, after the thermister RTH and the resistor R divide the power V2, the value of the generated sensing signal S1 will be higher than a preset voltage in the driving module 12, thereby directly making the motor 13 rotate at the full speed. Such a condition, in which the environment temperature is low but the rotating speed of the fan is kept at the full speed, not only wastes the energy but also deviates from the originally designed function.
Thus, it is an important subject to provide a fan system and a control device thereof capable of keeping a motor to rotate at a full speed to protect the fan system when a temperature sensing member of the fan system is abnormal, i.e. when the sensing member is detached or broken.